for a while now i have been wondering why you can have a yellow face parakeets, aka blue with a yellow face, but you can't have a green budgie with a white face. you would think the 4 "base" colors (blue/white and green/yellow) could be interchangeable seeming that the blue and yellow could 'mix' to make a yellow face. so why can't there be a green keet with a white face?

Well, to start there are only two base colors, yellow and white. Yellow (green budgie) is dominant in genes, while white (blue budgie) is recessive. Dominants can attach to recessives, but recessives can only in certain situations attach to dominants. White face is probably possible, but I don't think anyone's come across one in the parakeet/budgie world.
It can happen in cockatiels, so that's a start.
I'm not a gene master, but that's what I'd think....... see what I'm gettin' at?
I agree though, I'd love to see white face greens enter the mutation world!!!